Stem cells have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each stem cell has the potential to either remain a stem cell, a red blood cell, or a brain cell.
Stem cells have two important characteristics that distinguish them from other types of cell. First, they are unspecialized cells that renew themselves for long periods through cell division. The secon dis that under certain physiological or experimental conditions, they can be induced to become cells with special functions such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas.
There are three main types if stem c. being investigated for their potential use in research and medicine. They differ in their degree of differentiation and ability of self-renew.
- Embryonic Stem Cells: As their name suggest, these cells are derived from embryos. Specifically, embryonic stem c. are derived from embryos that developed from eggs that have been fertilized in vitro in an IVF clinic and then donated for research purposes with informed consent of the donors. They are not derived from eggs, fertilized in a woman’s body. The embryos from which human embryonic stem cells are derived are typically 4 or 5 days old and are a hollow microscopic ball of cells called blastocyst. The blastocyst includes three structures; the trophoblast, which is the layer of cells that surrounds the blastocyst; the blastocoel, which is the hollow cavity inside the blastocyst; and the inner cell mass, which is group of approximately 30 cells at one end of the blastocoel. Embryonic germ cells are derived from the part of a human embryo of foetus that will ultimately produce egg or sperms (gametes).
- 2. Adult Stem Cells: An adult stem c. is an undifferentiated cell found among differentiated cells in a tissue or organ, can renew itself, and can differentiate to yield the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Some scientists now use the term somatic stem cell instead of adult stem c. Unlike embryonic stem cells, which are defined by their origin (the inner cell mass of the blastocyst), the origin of adult stem cells in mature tissues is unknown.
In addition, umbilical cord blood cells are currently being used to treat a range of blood disorders and immune system conditions Stem c. that have the potential to develop into any of the cell types found in an adult organisms are called pluripotent. Embryonic stem cells are pluripotent.
Stem cells that only have the potential to make a few cells types in the body are called multipotent. Adult stem cells appear to be multipotent.
Cells that are capable of forming a completely new embryo that can develop into a new organism are called totipotent. A fertilized egg is totipotent. None of the stem cells used in research appear to have this capacity.
Potential Uses of Stem Cells
Stem cells have potential uses in many different areas of research and medicine.
Replace Damaged Tissue
Human stem c. could be used in the generation of cells and tissues for cell-based therapies (i.e., treating patients by transplanting specialized cells that have been grown from stem c. in the laboratory).
Due to their ability to replace damaged cells in the body, stem cells could be used to treat a range of conditions, including heart failure spinal injuries, diabetes and Parkinson’s disease. It is hoped that transplantation and growth of appropriate stem c. in damaged tissue will regenerate the various cell type if that tissue.
Study Human Development
Stem cells could be used to study early events in human development and how cells differentiate and function. This may help researchers find answers as to why some cells become cancerous and how some genetic disease develop, which may lead o clues as to how they may be prevented.
Testing of New Drugs
Stem cells grown in the laboratory may be useful for testing drugs and chemical before they are trained in people. The cells could be directed to differentiate into the cell types that are important for screening that drug. These cells may be more likely to mimic the response of human tissue to the drug being tested, compared to some of the animal models currently being used This may make drug testing safer, cheaper and more ethically acceptable to those who oppose the use of animals in pharmaceutical testing.
Stem c. may be useful potential toxins in substances, such as pesticides, before they are used in the environment.
Testing Gene Therapy Methods
Stem cells may prove useful during the development of new methods for gene therapy that may help people suffering from genetic illnesses.
These applications are all likely to be 10-20 years away.
Stem Cells and Cloning
The promise of stem c. to generate tissues for cell-based therapies is an exciting one. However, several hurdles need to be overcome before this can be realized.
Alternative Strategies for overcoming Immune Rejection
The current strategy for overcoming tissue rejection is the use of drugs that suppress the immune system. However, this means that the patient may then be more susceptible to infections and researchers are investigating new drugs that have fewer side effects.
An alternative strategy for overcoming immune rejection is to use adult stem c. from patients themselves to derive the transplant tissue.
Ethics of Stem C. Research
The overwhelming objection of stem cell research is that it involves the destruction of an embryo or foetus. For many, this constitutes destruction of a potential human and conflicts with religious and moral views held in our society. For others, the potential for this research to provide treatments and possible cures for debilitating illnesses that have no cure and significantly impact on our way of life override this concern. Central to any argument on this is what actually constitutes the beginning of life for a human. Opinions on this vary from the moment of conception, to a 14-day embryo, toa living baby at birth. This issue is highly emotive and it will always be necessary to consider all opinions and to balance the harm that might be done against the potential good this research may provide for those suffering from debilitating diseases.
The other major ethical issue associated with stem cell research ties in with the combination of embryonic stem cell and cloning technologies, leading to generation of an embryo that is a genetic clone of the donor of the nucleus. What is critically different in this context as opposed o that above is that an embryo is actually created for research or therapeutic purposes, and this raise a wider range of objections, in that a potential life is created for a specific purpose.
Also of concern here is the purpose of this cloning, which could be done purely for the purpose of generating tissue for transplantation. The embryo generated could be allowed to continue development and could potentially lead to the birth of a new human if implanted into a willing mother. There are serious ethical and medical concerns associated with the use of somatic cell nuclear transfer technologies to reproduce humans and it is illegal in Australia the UK and USA to conduct any research into reproductive cloning of humans.
Some people speculate that allowing any somatic cell nuclear ‘transfer will be the start of a slippery slope into reproductive cloning.
Permission for Stem-Cell Research
There are pluses and minuses associated with the research and use of all types of stem c. Which ones should research focus on?
The ethical issue surrounding the origin of embryonic stem c. will always be a sensitive issue. There are strict guidelines and legislation regarding any research involving embryos, but for many, research on adult stem c. is the only acceptable alternatives.
Embryonic stem and germ cells can give rise to every cell type in the body. Adult stem cell, however, are multipotent, giving rise to a limited range of cell types. This may limit their use in cell-based therapies and many researchers believe research using embryonic cells will be more fruitful. However, recent research has revealed that some adult stem cells may be able to generate different tissues under the right conditions and this may increase their therapeutic potential.
Embryonic stems have a greater capacity for self renewal and the cell lines that have been established will be useful for research into the effects of drugs and toxins and also into early human development. Their uncontrolled growth also leads to the development of tumours called teratomas, wghich may restrict their use in cell-based therapies. Research is continuing into ways to control and regulate the growth of ES cells more effectively. Embryonic germ and adult stem cells do not form these tumours in culture, which may make the better alternatives for transplant tissue sources.
Obviously, there are pros and cons to the use of all three type of stem c. and most scientists agree that it is important to pursue research into embryonic stem and germ cells and adult stem c. All scientists are aware that they must undertake their work ethically and within the bounds of the law and these can vary from country to country.